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(KTXL) — You will hear meteorologists warn that an atmospheric river is headed to California, but what type of weather is this?

In its basic form, it can best be described by its nickname, a “river in the sky.”

It is a giant vapor and cloud formation over the ocean near the tropics that makes its way hundreds of miles until it makes landfall, unleashing constant rain and snow for several hours or even days at a time, according to the National Oceanic and Atmospheric Administration.

The amount of rainfall and snowfall can cause localized flooding due to a large amount of precipitation that falls in one continuous flow, disrupting travel, causing damage to homes and potentially causing mudslides.

How do atmospheric rivers form?

According to NOAA, a column of condensed water vapor builds over the ocean and begins moving with the weather patterns.

These columns can be 250 to 375 miles wide, on average, and there is one present somewhere in the world at any given time, NOAA says.

Atmospheric rivers are not necessarily dangerous weather formations. Rather, they can benefit some areas, bringing in a large percentage of an area’s water supply and snowpack.

On the West Coast, atmospheric rivers bring between 30% and 50% of the annual precipitation, NOAA says.

Although most atmospheric rivers are “weak weather systems,” they are also responsible for the flooding risk in the areas where they create precipitation.

Atmospheric rivers in California

You may have heard about an atmospheric river under one of its nicknames in California, the “Pineapple Express,” named for the vapor formation that begins near Hawaii and makes its way to the West Coast, hitting the three Pacific states, California, Oregon and Washington.

A forecast from the National Weather Service from April 1, 2018, shows an atmospheric river.

These weather formations typically occur in late fall and throughout winter, and are a critical part of California’s water supply, NOAA says.

The Sierra Nevada, the mountain range that encompasses the majority of the eastern part of the state, receives as much as 75% of its snowpack from atmospheric rivers.

This snow later melts and fills the vast system of lakes and reservoirs that slowly release the water toward the agricultural Central Valley and the metropolitan areas of the state, primarily located along the coast and the interior.